1. Field of the Invention
The present invention relates to a decision feedback equalizer for suppressing intersymbol interference in the context of digital baseband communications over subscriber loops.
2. Description of the Related Art
In a conventional digital baseband transmission system using a metallic cable, a transformer is used at each end of the system for inductively coupling a data source to one end of the cable and a data sink to the other end of the cable for purposes of insulation and power feeding. Since the low frequency components of the signals transmitted are cut off by the transformers, the channel impulse response exhibits a waveform having a long tail following an impulse of transmitted energy. Such a long tail is the potential source of undesirable intersymbol interference. Decision feedback equalization has been in widespread use for suppressing intersymbol interference in the context of high-speed digital communications. A decision feedback equalizer currently in use comprises a subtractor to which an input digital signal is applied and in which the intersymbol interference component of the signal is compensated for by subtracting from it the output of a nonrecursive, finite impulse response filter. A binary decision is made on the output of the subtractor by a decision circuit to produce a logical 1 or 0 by comparison with a threshold value and a decision error is detected and the tap gains of the nonrecursive filter are updated with correlations between the decision error and the tap signals of the filter. However, a large number of delay taps is required for the complete elimination of intersymbol interference component with a long, gradually decaying tail.
U.S. Pat. No. 5,084,865, S. Koike, describes an echo canceller comprising a nonrecursive filter and a first-order recursive filter. However, in transmission systems where transformers are provided one for each end of a transmission line, the receiving end of the line suffers from the second-order effect of the lowpass cut-off characteristic of the transmission line that arises from the transformer at the transmit end. The use of the first-order recursive filter is inadequate for complete elimination of a long tail portion of intersymbol interference.
The use of a second-order recursive filter in addition to a nonrecursive filter is disclosed in a paper "Pole-Zero Decision Feedback Equalization with a Rapidly Converging Adaptive IIR Algorithm", P. M. Crespo et al, IEEE Journal on Selected Areas in Communications, Vol. 9, No. 6, Aug. 1991, pages 817-829. In this publication, two coefficients of variable quantity are involved in the IIR algorithm. However, because of the use of more than one coefficient of variable quantity the proposed IIR algorithm is complex and difficulty will be encountered in implementing the IIR algorithm.